In the art of making laminates of cellulosic substrates, it has long been appreciated that phenol-aldehyde resins tend to produce laminates of cellulosic materials which have relatively poor cold punchability characteristics and have poor flame resistance.
Heretofore, attempts have been made to use compositions of resole resins with plasticizers and flame retardants alone or in combination to produce laminates having punchability and flame resistance, but the results have suffered from a number of disadvantages. These compositions gave slow treating speed and much smoke when the impregnated paper was passed through an oven to drive off solvents. When this treated paper was made into laminates, they frequently had undesirably high water absorption, low flexural strength, surface exudation, erratic burning characteristics and poor electrical properties.
It has now been discovered that certain compositions comprising a phenol-aldehyde resole resin and a halogen containing interpolymer can be used to produce laminates incorporating cellulosic substrates in sheetlike form which not only give fast treating prepreg, with desirable water absorption and flexural strength but no surface exudation, good fire-smoke retarding characteristics, good electrical properties and excellent cold punchability.
This combination of a normally thermoplastic interpolymer and a thermosettable resin is typically in the form of an aqueous system in which the resole portion is dissolved while the halogenated interpolymer portion is suspended in the form of a latex. This combination is unusual because it is water dilutable or organic liquid dilutable. Laminates can be made by one-pass treatment avoiding a two-step impregnation process.
Cold punchable laminates are important because energy is not needed for heating prior to punching into household appliances and terminal boards, structural parts, electrical switches, instrument panels, insulating washers, switch parts, jack spacers and so forth.